Alaska is rich with permafrost and approximately 75 percent of the pipeline passes through permafrost terrain. Permafrost depth along the pipeline ranges from a few inches to more than 2,000 ft. The pipeline traverses the Continuous Zone on the North Slope and Brooks Range. It then encounters the Discontinuous and Sporatic Zones and passes through areas of no permafrost in the immediate vicinity of Valdez.
By definition, permafrost is any rock or soil material that has remained below 32¡ F continuously for two or more years. There are five different types of permafrost: cold, ice-rich, thaw-stable, thaw-unstable and warm permafrost. The two-year minimum stipulation is meant to exclude the overlying ground surface layer, which freezes every winter and thaws every summer. This layer is called either "active layer" or "seasonal frost." Problems that arise as a result of permafrost existence are frost heaving, frost jacking and thaw settlement.
Pipeline design is based primarily on the soil conditions encountered along the right-of-way. The three principal pipeline design modes are above-ground, below-ground, and special burial. Where thaw-unstable permafrost exists, problems associated with melting permafrost were avoided by placing the pipeline above ground on an elevated support system known as Vertical Support Members (VSM). In areas where either unfrozen or thaw-stable permafrost were encountered, the pipeline was buried in the conventional manner. Special burial below-ground sites are found in areas where thaw-unstable permafrost was encountered but where the pipeline had to be buried for highway, animal crossings, or to avoid rockslides and avalanches.